This invention relates to electron discharge device cathodes and particularly to fast warmup cathodes. While not limited thereto, it has particular application in the multiple gun structure of color cathode ray tubes employed in color television receivers.
Conventional cathode ray tubes of the color variety are provided with multiple electron guns each of which contains an electron emitting cathode. The cathodes are usually indirectly heated; i.e., they comprise a more or less tubular cathode assembly with a closed end having an insulated heater therein to provide the heat necessary to cause an emissive material to emit electrons.
The cathodes normally employed in color cathode ray tubes conventionally have a warmup time of 12 to 15 seconds; i.e., it requires that long a time for sufficient electrons to be present from the cathode to be drawn to the anode and establish a raster on the screen of the tube. These warmup times have been considered to be detrimental to the viewing public in that it requires a long wait from turn-on to an acceptable or viewable picture on the tube. In the past, this detrimental condition has been obviated by the provision of an "instant on" feature provided by some television receiver manufacturers. With this feature a raster or viewable picture is obtained on the picture tube almost instantaneously with the turn-on of the set. However, in the past this feature has not been accomplished by a fast warmup cathode but rather by a bleeder current which constantly maintains the cathode heater at a near normal operating temperature. Thus, in effect, the cathode ray tube is never completely turned off. When the television receiver is a complete tube version containing many receiving tubes or a hybrid version including some tubes and some solid state devices, the bleeder current of the "instant on" feature is applied also to the heaters of the other receiving tubes within the set. This condition has been alleged to provide a dangerous fire hazard in some receivers. It is also quite wasteful of electricity since, as mentioned above, the receiver is never completely turned off and the set is constantly drawing electrical power.
Fast warmup cathodes have been proposed as the solution to this problem; however, many of the proposed types have been either extremely difficult to build or have been very expensive or have required considerable design changes in the conventional electron gun structures.
One construction that has obviated some of these problems is shown in U.S. Pat. No. 3,881,124. An improvement to the cathode shown in the 3,881,124 patent is described in U.S. Pat. application Ser. No. 494,640, filed Aug. 5, 1974, now U.S. Pat. No. 3,919,751 as a division of Ser. No. 440,685, filed Feb. 8, 1974 now abandoned. The above recited patent and applications are assigned to the assignee of the present invention. In Ser. No. 494,640 it is taught that the warmup time can be decreased by employing a black, high heat radiating material on the inside of the cathode. A suggested material is an alloy comprised essentially of about 20% chromium with the remainder being nickel. To transform this material to a black coating it is fired in wet dissociated ammonia for about 10 minutes at at least 900.degree. C and preferably at 1200 to 1300.degree. C. Herein lies a problem; namely, while individual cathodes can be manufactured by this technique, the high firing temperatures can cause diffusion bonding between cathodes when mass production is employed. Accordingly, it would be an advance in the art if the above problems could be avoided.